In a retainer-type lead storage battery, the electrolyte is limited in amount and the negative electrode has capacity larger by 10-30% than that of the positive electrode. Accordingly, when the battery is charged, the positive electrode is fully charged before the negative electrode. Oxygen generated from the positive electrode due to overcharge is absorbed and consumed by the negative electrode.
After assembly, lead storage batteries of this type are generally subjected to forming and then delivered or preserved in a full-charge state. Until they are put to actual use, it is required that they are supplementarily charged at regular intervals, for example every six months and preferably every three months, in order to preserve such batteries at all times substantially in a full-charge state.
Lead storage batteries generally undergo self-discharge of about 0.1-0.15% of the battery capacity for every day, thereby to reduce their capacity due to self-discharge of 20-30% for a period of six months.
If a discharge product produced by self-discharge of a battery can be activated by charging or if the battery capacity can be restored, it is not required to supplementarily charge the battery so often. However, when a battery is left in a self-discharge state for a long period of time, a discharge product or lead sulfate (PbSO.sub.4) is inactivated, thereby to reduce the charging efficiency. Accordingly, the capacity cannot be sufficiently restored to provoke a decrease in battery characteristic, thus requiring frequent maintenance of supplementary charge. Such maintenance is not only troublesome, but also includes the problem that the battery capacity cannot be fully restored even with repeated supplementary charge.
FIG. 1 is a graph of self-discharge characteristics showing the capacity rates to an initial battery capacity of the battery capacity where the batteries were discharged at room temperature (20.degree. C.) after leaving for various periods of time under such discharging condition that they were discharged with a current corresponding to 0.1 c and stopped discharging when the voltage of battery was reached 1.7 V.
FIG. 2 is a graph of the capacity reset characteristics showing the capacity reset rates to an initial battery capacity of the battery capacity where the batteries were charged with a constant voltage (2.5 V) for 16 hours and then discharged with the above discharge condition, after leaving for various periods of time.
In FIG. 1 and FIG. 2, the initial capacity of battery which was discharged immediately after forming under the above discharge condition, is defined as 100%.
According to the studies of the inventor and other, batteries are slowly decreased in voltage by self-discharge when they are left after subjected to forming. With such self-discharge, there are chemically bonded Pb.sup.++ ions in the corrosion layer of lead dioxide (PbO.sub.2) on the surface of the collector member or Pb.sup.++ ions of the active material PbO.sub.2 with SO.sub.4.sup.-- ions in the H.sub.2 SO.sub.4 electrolyte, thereby to produce lead sulfate (PbSO.sub.4). When batteries are left for a long period of time, such lead sulfate (PbSO.sub.4) easily becomes inactivated. This decreases the charging efficiency when batteries are re-charged, so that the capacity cannot be fully restored to deteriorate the battery characteristics. Such deterioration becomes greater as the supplementary charging cycle period is longer or batteries are left for a longer period of time. Such deterioration is further accelerated when batteries are left in a high temperature atmosphere.